The present invention generally relates to electric power transmission devices. More particularly, the invention relates to apparatus and methods for preventing equipment failures that are signified by overheated dielectric insulation oil.
Electric power transmission devices such as transformers and switch gear are often immersed in a specially compounded oil having dielectric properties for purposes of insulation, isolation and cooling. On occasion, these devices generate extremely high operating temperatures. Although the oil will not burn in the absence of atmosphere or oxygen, small portions will directly decompose under the intense heat of electrical arcing. This is especially true for load tap changers and similar switchgear wherein mechanical contact switches are routinely closed and opened with a high potential difference standing at the switch points. Such decomposition transforms the oil into elemental carbon, which remains in the remaining oil reservoir as suspended graphite particles.
With respect to equipment having pressure sealed oil reservoirs, the internal reservoir pressure increases as the oil heats under load and intense switching activity. Over time, seals and gaskets weaken to release the temperature induced pressure load by release of oil volume. Dielectric oil is believed to be an environmental hazard. Consequently, oil loss by leakage is to be avoided. Equally damaging, however, is the consequence of oil volume losses due to temperature induced pressure. When the electrical load activity causing the temperature/pressure increase subsides, the oil cools, contracts and depressurizes. To the extent that oil volume is expelled from the reservoir under high pressure, the void left by the displaced oil volume is filled under cool, negative pressure from the surrounding atmosphere. This atmosphere might enter the system through the same seal and gasket weaknesses that release oil from the reservoir in the first place. Carried with such induced atmosphere into the oil reservoir is water vapor. Since the chemical nature of the oil is hydrophilic, any atmospherically carried water coming into surface contact with the oil is adsorbed and entrained.
Unpressurized oil reservoirs are vented to the atmosphere. The operative consequence of an atmospheric vent is to admit atmospherically borne water vapor. By the same mechanism as previously described, water is adsorbed and entrained with the dielectric oil in unpressurized reservoirs.
Both, water droplets and graphite particles are intolerable contaminants of the oil and must be removed, either periodically or continuously. Fortunately, both contaminants are effectively removed by relatively simple, depth wound, unsized paper reel filters. A traditional load tap changer installation often will connect the oil reservoir for the load tap changer by external plumbing conduits to adjacently housed pump and filter units. Circulation around the conduit loop is driven by the pump motor which is controlled by cycle timers and filter pressure differential monitoring switches. Circulation may be continuous or intermittent, depending on the type of transformer or the service to which it is applied.
Transformer reservoirs are not usually filtered, or externally circulated while the transformer is operative or "on line". External filtration of the transformer oil may be performed by a portable apparatus that is connected to the oil reservoir for oil circulation through filtration devices for a predetermined time interval while the transformer is off-line. At the end of the filtration time interval, the portable circulation apparatus is disconnected and removed.
Due to the absence of switch contact activity within a transformer, a large reservoir volume and reservoir case fins normally keep the load induced temperature fluctuations of transformers within tolerable ranges. Although the oil reservoirs of load tap changers and similar switchgear are substantially smaller, the respective oil temperatures should remain substantially the same. The heat exchange rate of switchgear can usually be matched to the induced heat rate of load switching by pumped, external circulation and filtration. By engineered design, therefore, dielectric oil temperatures respective to transformers and associated switchgear are operationally matched and not normally perceived as a controlled parameter.
However, externally circulated switchgear oil, proportionately, has a considerably greater risk of loss and contamination due to the greater number of conduit connections and dynamic fluid seals.
It is, therefore, an object of the present invention to actively monitor the switchgear oil temperature.
Another object of the invention is to provide an excess oil temperature alarm signal system that is activated by one or more temperature sensors positioned in an oil filter circulation system that serves a transformer load tap changer.
Also an object of the present invention is an oil temperature monitoring system for transformer switchgear that disconnects the associated transformer from its load circuit when the switchgear oil temperature exceeds an acceptable limit.
A still further object of the invention is a system for monitoring the oil temperature differential respective to a transformer and the switchgear having a control association with the transformer.